Use the "real" signature of the foreign procedure in `ExecFpi` op

I can't help but feel it would be better if we could somehow use the "real" signature of the foreign procedure somehow, probably by storing the type signature in an attribute and making the operands/results `AnyType` - but I see that more as a future improvement - this will do for now I think.

_Originally posted by @bitwalker in https://github.com/0xMiden/compiler/pull/1113#discussion_r3400376854_

Look at the ExecFpi op implementation and the foreign procedure handling in the compiler. The issue suggests storing type signatures in an attribute and changing operands/results to AnyType. Start by examining the PR #1113 discussion and the current code around foreign procedures to understand the context.

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